1. Field of the Invention
The present invention relates to a conductor paste used for forming circuitry on a ceramic substrate, and a circuit substrate having electric circuit formed thereon using the conductor paste.
2. Technical Background
Circuit substrates made of a ceramic, such as alumina (Al2O3) or aluminum nitride (AlN), are commonly used as circuit substrates for various electronic components.
As one of the methods for forming the circuit (a conductor layer) on such as a ceramic substrate, a conductor paste is used to form the electric circuit though the process of screen printing, drying and firing.
The conductor paste is generally one in which a conductive powder and a glass powder are dispersed in an organic medium. For example, in JP-A-10-340622, there is disclosed a conductor paste in which a conductive powder comprises silver and palladium, a glass powder has an average particle diameter of not more than 1.5 μm and a softening point in a range of from 750 to 900° C., and a silicon dioxide powder has an average particle diameter of not more than 0.1 μm are dispersed in an organic vehicle.
In US 2004-0245508, there is disclosed a conductor paste for an aluminum nitride substrate comprising a boron-containing compound and a metal oxide, this structure being designed to increase adhesive strength.
Meanwhile, in recent years, to reduce environmental impact, disuse of lead and of cadmium has been encouraged. Also, to keep down manufacturing cost, it is desirable to reduce the firing temperature (to not more than 650° C.). Note that the firing temperature to form a conductor film on a ceramic substrate has generally been approximately 850° C.
Retaining adhesion is important. It is not enough only to use glass frits having low softening points in the paste to promote good adhesion where the firing temperature is lower than 650° C. One problem that may occur at such low temperatures is that sintering may be deficient and strong bonds may not occur, limiting adhesion.
It is an object of the present invention to provide a conductor paste that is lead-free and cadmium-free, can be fired at a low temperature of not more than 650° C., and has excellent adhesive strength on a ceramic substrate. Further, it is an object of the present invention to provide an electric circuit substrate in which the adhesive strength between a ceramic substrate and a conductor film is adequate.